multi: Fix final hop payload size for AMP payments.

routing/pathfind.go

@@ -408,6 +408,11 @@ type RestrictParams struct {
 	// invoices.
 	PaymentAddr *[32]byte
 
+	// Amp signals to the pathfinder that this payment is an AMP payment
+	// and therefore it needs to account for additional AMP data in the
+	// final hop payload size calculation.
+	Amp *AMPOptions
+
 	// Metadata is additional data that is sent along with the payment to
 	// the payee.
 	Metadata []byte
@@ -1134,12 +1139,21 @@ func lastHopPayloadSize(r *RestrictParams, finalHtlcExpiry int32,
 		mpp = record.NewMPP(amount, *r.PaymentAddr)
 	}
 
+	var amp *record.AMP
+	if r.Amp != nil {
+		// The AMP payload is not easy accessible at this point but we
+		// are only interested in the size of the payload so we just use
+		// the AMP record dummy.
+		amp = &record.MaxAmpPayLoadSize
+	}
+
 	finalHop := route.Hop{
 		AmtToForward:     amount,
 		OutgoingTimeLock: uint32(finalHtlcExpiry),
 		CustomRecords:    r.DestCustomRecords,
 		LegacyPayload:    legacy,
 		MPP:              mpp,
+		AMP:              amp,
 		Metadata:         r.Metadata,
 	}
 

routing/pathfind_test.go

@@ -3469,6 +3469,7 @@ func TestLastHopPayloadSize(t *testing.T) {
 		)
 		_, blindedPoint       = btcec.PrivKeyFromBytes([]byte{5})
 		paymentAddr           = &[32]byte{1}
+		ampOptions            = &AMPOptions{}
 		amtToForward          = lnwire.MilliSatoshi(10000)
 		finalHopExpiry  int32 = 144
 
@@ -3510,6 +3511,7 @@ func TestLastHopPayloadSize(t *testing.T) {
 				PaymentAddr:       paymentAddr,
 				DestCustomRecords: customRecords,
 				Metadata:          metadata,
+				Amp:               ampOptions,
 			},
 			amount:         amtToForward,
 			finalHopExpiry: finalHopExpiry,
@@ -3524,6 +3526,7 @@ func TestLastHopPayloadSize(t *testing.T) {
 				PaymentAddr:       paymentAddr,
 				DestCustomRecords: customRecords,
 				Metadata:          metadata,
+				Amp:               ampOptions,
 			},
 			amount:         amtToForward,
 			finalHopExpiry: finalHopExpiry,
@@ -3560,6 +3563,13 @@ func TestLastHopPayloadSize(t *testing.T) {
 				)
 			}
 
+			// In case it's an AMP payment we use the max AMP record
+			// size to estimate the final hop size.
+			var amp *record.AMP
+			if tc.restrictions.Amp != nil {
+				amp = &record.MaxAmpPayLoadSize
+			}
+
 			var finalHop route.Hop
 			if tc.restrictions.BlindedPayment != nil {
 				blindedPath := tc.restrictions.BlindedPayment.
@@ -3586,6 +3596,7 @@ func TestLastHopPayloadSize(t *testing.T) {
 					OutgoingTimeLock: uint32(tc.finalHopExpiry),
 					Metadata:         tc.restrictions.Metadata,
 					MPP:              mpp,
+					AMP:              amp,
 					CustomRecords:    tc.restrictions.DestCustomRecords,
 				}
 			}

routing/payment_lifecycle.go

@@ -628,7 +628,7 @@ func (p *paymentLifecycle) createNewPaymentAttempt(rt *route.Route,
 		return nil, err
 	}
 
-	// It this shard carries MPP or AMP options, add them to the last hop
+	// If this shard carries MPP or AMP options, add them to the last hop
 	// on the route.
 	hop := rt.Hops[len(rt.Hops)-1]
 	if shard.MPP() != nil {

routing/payment_session.go

@@ -258,6 +258,7 @@ func (p *paymentSession) RequestRoute(maxAmt, feeLimit lnwire.MilliSatoshi,
 		DestCustomRecords:  p.payment.DestCustomRecords,
 		DestFeatures:       p.payment.DestFeatures,
 		PaymentAddr:        p.payment.PaymentAddr,
+		Amp:                p.payment.amp,
 		Metadata:           p.payment.Metadata,
 	}